The invention relates to a process for reducing the foam formation when a dispersion or liquid is treated with steam.
In the emulsion polymerization of unsaturated compounds, generally, polymerization is not carried out to the quantitative conversion of the monomers. Frequently, the polymerization is also intentionally terminated at a lower conversion rate. In all cases, in addition to the residual amounts of monomers, the accompanying components of the monomers used and the auxiliaries, for example the emulsifiers used, as well as the low-molecular-weight byproducts which are produced during the polymerization remain in the dispersions. These unwanted volatile organic components which remain in the product are given the collective name xe2x80x9cresidual volatilesxe2x80x9d below. In the commission decision 96/13/EC for establishing the ecological criteria for the award of the EC eco-label to indoor paints and varnishes of Dec. 15, 1995, these residual volatiles are divided into volatile organic compounds (VOC) and volatile aromatic hydrocarbons. In both cases, these are organic compounds having a boiling point (or initial boiling point) of at most 250xc2x0 C. under normal pressure conditions. The volatile aromatic hydro-carbons in this case have at least one aromatic nucleus in the structural formula. The collective term xe2x80x9cresidual volatilesxe2x80x9d used here denotes all such organic compounds having a boiling point (or initial boiling point) of at most 250xc2x0 C.
In addition to dispersions, liquids can also comprise residual volatiles. For example, when a dispersion formed by emulsion polymerization is broken by an electrolyte or acid, at least a portion of the residual volatiles remains in the liquid which separates off from the polymer. The residual volatiles can be a problem in environmentally appropriate disposal of such liquids. Furthermore, residual volatiles are unwanted in many applications of dispersions or suspensions, for example in the food or cosmetics sector or for indoor applications, and attempts are made to remove these as completely as possible.
Dispersions or liquids are therefore subjected to a treatment which removes the residual volatiles. This treatment is termed deodorization. Various processes and apparatuses are known for this. In addition to chemical processes, which generally only affect the unsaturated compounds, however, these are mainly stripping processes, in which a stripping gas is passed through the liquid or dispersion. It has long been known to remove residual volatiles by passing steam through the previously introduced heated dispersions or liquids. Carrying out this steam treatment poses great difficulties in the case of many polymer dispersions, particularly those having a surface tension below 40 dyne/cm, because of their content of emulsifiers which cause vigorous foaming of the dispersion. The generally large amount of foam, dependent, inter alia, on the type and concentration of the emulsifiers and other polymerization auxiliaries present, on the type and amount of the monomers present as well as on the particle size of the polymers, makes the process slow, laborious and frequently impossible to carry out. Although adding certain defoamers, such as complex alkali metal phosphates in combination with salts of polyvalent metals, allows the foaming to be decreased, addition of such defoamers is frequently undesirable from the aspect of the stability and application properties of the dispersions. For this reason, the addition of defoamers is usually avoided. Instead, for example, a steam/dispersion foam mixture is subjected to a rapid pressure drop at a high flow velocity. To destroy foam, in this case, one or more nozzle-shaped constrictions in pipe cross section and impingement or deflection baffles or similarly acting internals may be used in the paths of the steam/dispersion foam mixture. In the preferred embodiment, the steam/dispersion foam mixture is introduced tangentially into an evacuated separator at high velocity after passage through a nozzle.
The liquids or dispersions preferably used in the processes according to the invention shall now be listed below:
Liquids
The liquids used in the process according to the invention may be any liquids having a removable residual volatile content. Preferably, aqueous solutions or liquids having a tendency to foam formation are used. Examples of such liquids can be solutions which arise after separating off the polymer from suspension or emulsion polymers, or other foam-forming liquids which comprise unwanted volatile components, in particular if these liquids serve as solvents. Other examples which may be mentioned are polymer solutions, in particular aqueous polymer solutions. After preparation in the appropriate solvent, these frequently still comprise residual monomers and other components which can be removed by deodorization.
Dispersions
The dispersions used in the process according to the invention may be any dispersions which have removable residual volatile contents. Examples of such dispersions can be dispersions of contaminated soils, dispersions of inorganic particles, dispersions of biomolecules and, preferably, dispersions of organic compounds, in particular polymer dispersions. The dispersions are preferably aqueous dispersions.
In typical deodorization processes, the dispersion or liquid is fed to the top of a column, while steam is introduced in the lower region of the column. The dispersion or liquid to be deodorized passes through the column in countercurrent to the steam. In a countercurrent column of this type, the pressure at the top is reduced with respect to that at the bottom. During the deodorization of aqueous dispersions in particular polymer dispersions in deodorization columns where reduced pressure is generated at the column top, the dispersion to be deodorized foams. This very rapidly leads to complete filling of the column top with foam. When the column top is full of foam, the foam is carried along by the stripping vapor into the following condenser. Since this does not occur uniformly, but in surges, pressure fluctuations occur at the column top and ultimately in the entire column. This interferes with the steady-state operation of the column. Consequences of the pressure fluctuations range from backmixing in the column to weeping of the plates. This considerably reduces the efficiency of the columns. Moreover, in the case of vigorous foams, product can pass into the condenser and there can be lost via the discharge into the effluent. If product passes into the condenser, it is usually also further sucked into the vacuum pump and conducted by this into the exhaust gas system. If this occurs repeatedly, this can lead to destruction of the vacuum pump and blockage of the exhaust gas lines. Both necessitate plant downtimes with corresponding losses in turnover.
To avoid condenser and vacuum system filling with foam, it is known to install, between column top and condenser, a foam separator having foam destruction by means of a pressure jump (DE 12 48 943). This foam destruction by means of the pressure jump requires a sufficient pressure difference between column top and foam separator. In the case of stable, vigorous foam development, as is customary in circulating deodorization, this system generally brings good results. However, if, as is the case in deodorization columns, a foam surge and pressure fluctuations in the system are to be expected, a pressure difference necessary for reliable foam destruction is frequently not achieved. As a consequence thereof, the foam destruction does not function reliably. This is accompanied by the abovementioned disadvantages. These disadvantages have not been overcome hitherto even by means of a large and thus expensive foam separator as an additional apparatus.
It is an object of the present invention to improve the above-described process in such a manner that, during the feeding of foam- forming dispersion or liquids into the top of a deodorization column, the foam formation is reduced.
We have found that this object is achieved by the process for reducing the foam formation when a dispersion or liquid is treated with steam by
a) introduction of steam into the lower region of a column and simultaneous feed of a medium at the top of the column, the steam ascending to the top of the column,
b) contacting the medium with the steam, with foam formation.
The process according to the invention comprises the medium fed in stage a) having been generated by mixing a dispersion or liquid with steam in an apparatus, this medium during the feed into the top of the column having a higher temperature than that of the steam which comes into contact with the medium (in stage b)).
It has been found, surprisingly, that the reduction of the amount of foam occurring in the top of the column is achieved by the direct introduction of steam into the dispersion stream or liquid stream fed to the column. In more precise studies it was found that the dispersion or liquid, during feed to the column top, foams particularly vigorously when it is colder than the water vapor temperature at the column top which is established at the column pressure. Generally, the foaming can only be sufficiently reduced by, according to the invention, introducing into the dispersion or liquid sufficient hot direct steam to heat the medium, the dispersion/steam mixture or liquid/steam mixture, which is introduced into the column top, adequately (i.e. above the temperature of the steam at the column top). This is achieved according to the invention by means of a nozzle which introduces steam into the dispersion or liquid, into the vertically upright inlet tube to the column top at a sufficient distance therefrom. For this purpose, preferably, a ring-gap nozzle is used. In order that this steam is uniformly distributed in the dispersion or liquid and heats this up, horizontal plates having large orifices are provided as steam distributors in the pipe on the path up to the inlet into the column top. By this means, a backpressure is built up by at least one plate having an orifice as baffle plate. The baffle plate can also be replaced or supplemented by a diaphragm valve as adjustable throttle.
Further details and advantages of the invention can be taken from the examples which are described with reference to the drawing.